U.S. Pat. No. 3,452,313 (prior art document 1) discloses a protector for a three-phase motor, of the above-described type, for example. The protector described in prior art document 1 comprises a thermally responsive plate including a bimetal provided in a hermetic container comprising a metal housing and a metal plate. Three conductive terminal pins are hermetically fixed to the metal plate. The terminal pins have respective one ends which protrude in the hermetic container and to which fixed contacts are secured respectively. On the other hand, movable contacts are secured to portions of the thermally responsive plate opposed to the fixed contacts respectively. The movable and fixed contacts constitute three pairs of switching contacts. Furthermore, the thermally responsive plate as described above has a centrally located through hole and is disposed in the hermetic container with a support (a bolt or the like) being inserted into the hole by swaging or screwing, as disclosed by Japanese Patent Application Publication No. JP-A-H01-279532 (prior art document 2).
The protector as described above is mounted in a hermetic housing of a hermetic electrical compressor thereby to be used as a thermal protector for a three-phase electric motor for driving the compressor, for example. In this case, the aforementioned three terminal pins are connected to neutral point side terminals of the phase windings respectively. A temperature around the thermally responsive switch exceeds a predetermined value when a temperature of refrigerant in the hermetic compressor rises to an unusually high temperature or when an abnormal current flows into the three-phase motor. The thermally responsive plate is reversed thereby to open the contacts. As a result, the AC current flowing into the three-phase motor is interrupted. Furthermore, when the temperature drops to or below the predetermined value, the thermally responsive plate returns to the former state such that the contacts are re-closed, whereupon the three-phase motor is re-energized.